This relates to multilevel memories.
Multilevel memories are memories that store more than two states. Multilevel memories may be used in a variety of memory technologies, including flash and phase change memories.
In some flash memories, there are two distinct states in which the cell is either conducting or non-conducting. Intermediate states between those cells may also be defined.
The number of states that may be achieved in one cell and the margin between states is a function of the available threshold voltage range. Basically, the two different primary states have different threshold voltages and the number of intermediate states between those threshold voltages may be determined by the range or distance between them. Normally, that range or distance must be sufficient to adequately distinguish the intermediate states.